1. Field of the Invention
The present invention relates to measurement of electromagnetic wavefronts. More particularly, the invention pertains to quantitative, instantaneous measurement of strongly converging optical wavefronts.
2. Description of the Related Art
Wavefront measurements are important in the manufacture of many optical components, including optical data-storage laser heads. The trend towards high-numerical-aperture laser systems for high-density storage makes wavefront measurements particularly difficult.
A conventional measurement of the wavefront quality of a light beam may employ spatial filtering of a small portion of the source light to produce a spherical reference wave that is subsequently combined with the original wavefront to produce an interferogram. As is well understood in the art, the intensity pattern generated by the interference yields fringes of constant phase difference that can be analyzed to evaluate the quality of the light beam. However, care must be taken not to introduce aberrations while dividing the original beam and recombining it with the reference beam. Therefore, these optical measurement systems must be carefully calibrated to remove artifacts from the optical paths of the beams.
This is especially important in systems that do not utilize a common optical path arrangement. In particular, for strongly converging beams, such as utilized in testing of DVD pick-up heads and of high-numerical-aperture photolithographic equipment for the semiconductor industry, a high-numerical-aperture point reference source must be generated externally to the system and used for calibration and correction of optical-path errors. Such a high-numerical-aperture point reference source is difficult to construct and maintain in proper alignment under practical manufacturing and test conditions. A system capable of producing a test wavefront and a reference wavefront from the same high-numerical-aperture source beam in a common-path arrangement would be very desirable because it would eliminate the prior-art need for an external point reference calibration source.
Common-path interferometry that takes advantage of a so-called point-diffracting element is a simple, well known configuration for measuring the quality of an optical wavefront. It was first described nearly 70 years ago by Linnik (see R. N. Smart and W. H. Steel, “Theory and Application of Point Diffraction Interferometers,” Jpn. J. Appl. Phys., 14 351–356, 1975). Early common-path designs produced only a single optical interference pattern, which made it difficult to obtain quantitative information about the wavefront under test. Recently, significant research has been devoted to adapting phase-shift interferometric techniques to common-path interferometry in order to improve the precision of wavefront measurements.
Providing more than one value of phase shift between the object beam (also referred to as the test beam) and the reference beam has proven to be difficult in a common-path design. Several methods have been implemented, however, using some form of temporal phase shifting (wherein the phase shift between the reference and test waves is introduced sequentially.) For example, a liquid crystal waveplate with a microsphere point diffractor was disclosed in U.S. Pat. No. 5,689,314, and rotating waveplates and polarizers with small pinholes were described in U.S. Pat. Nos. 4,762,417 and 4,575,247. The prior art also describes using point-diffraction interferometers that have non-common paths (U.S. Pat. No. 4,744,658) and nearly common paths (U.S. Pat. No. 5,835,217).
These methods demonstrated a high degree of accuracy (on the order of one fortieth of a wavelength). Yet, they are restricted to low-numerical-aperture beams because of the limited feature size of the phase-plate. In addition, the optical thickness of the retardation and splitting elements adds aberrations that must be subtracted through calibration in order to obtain accurate measurements. Also, the temporal nature of phase-shifting techniques requires a high degree of mechanical stability of the pinhole and the interferometer with respect to the test beam during the entire acquisition time (typically, 3 to 7 video frames), thus rendering the technique particularly sensitive to vibrations.
In polarization interferometers (where the test and reference wavefronts have orthogonal polarizations), phase shifting of interferograms is accomplished by sequentially introducing a phase step between the test and reference waves (temporal phase shifting), such as with an electro-optic modulator, or by splitting the optical path into parallel channels and introducing simultaneous phase steps (spatial phase shifting). Spatial phase shifting allows data acquisition speeds that are several orders of magnitude faster than possible with temporal phase shifting, thereby providing significant vibration immunity and improved throughput.
Several methods of spatial phase shifting have been disclosed in the prior art. Smythe and Moore (1983) and Koliopoulos (1993) describe a spatial phase-shifting approach wherein a series of conventional beam splitters and polarization optics are used to produce three or four phase shifted images onto one or more cameras for simultaneous detection. Several U.S. patents (U.S. Pat. Nos. 4,575,248, 5,589,938, 5,663,793, 5,777,741 and 5,883,717) disclose variations of this method where multiple cameras are used to detect multiple interferograms. Several prior-art publications (for example, Hettwer et al., “Three channel phase-shifting interferometer using polarization-optics and a diffraction grating,” Opt. Eng. 39(4), 960, 2000) and patents (U.S. Pat. Nos. 4,624,569 and 6,304,330) describe techniques wherein three or more interferograms are simultaneously imaged onto a single sensor. Other publications (U.S. Pat. Nos. 5,155,363 and 5,361,312) refer to methods where quantitative measurements can be made on a single CCD sensor by introducing a significant tilt between the reference and test wavefronts.
It is also known from the prior art that arrays of long conducting strips with periods much less than the wavelength of light can be used as polarizing elements. These arrays efficiently transmit light with polarization orthogonal to the strip direction while reflecting light with a collinear polarization (see, for example, U.S. Pat. Nos. 6,108,131, 6,122,103, 6,208,463 and 6,243,199). The planar nature of such a conducting strip structure permits using it as a polarizer over an extremely wide angle of incidence and over a broad range of wavelengths (provided that the array period remains much less than the wavelength). Earlier prior-art patents considered the effects of arrays of strips that were very long in relation to the optical wavelength of the system. However, later research (A. Jenson et al., “Finite-Aperture Wire Grid Polarizers,” J. Opt. Soc. Am., December 2000, 2191–2198) showed theoretically that sub-wavelength wire-grid arrays could provide a high degree of polarization extinction even when the length of the wire structure is only on the order of half a wavelength.
It is also known from the prior art that it is possible to fabricate polarizers having a thickness of a few microns or less using thin-film nanomaterials (see U.S. Pat. No. 6,174,394). These films have also excellent performance as a function of incident angle. Therefore, they may be used as polarizing elements in a fashion equivalent to conductive grid structures.
This disclosure illustrates how a thin polarizer arrangement (such as a sub-wavelength, conducting array pattern or a thin nanomaterial film with a finite aperture) can be used as a point diffraction filter in combination with simultaneous phase-shift interferometer configurations to produce a system capable of high-precision single-pulse measurement of wavefronts over a wide range of numerical apertures.